1. Field of the Invention
This invention generally relates to spinal bone implants.
2. Prior Art
Spinal bone implants are used for treating a variety of spinal conditions. A typical implant is comprised of pedicle screws inserted into the pedicles of vertebral bodies, and a rigid rod secured to the screw heads. Each pedicle screw is comprised of a U-shaped seat with a hole at the bottom and a slot between its arms, a screw extending through the hole in the seat, and a nut threaded onto an external thread around the seat. This is seen in the design of Puno et al. The implant is inserted by drilling holes in the top end of the pedicles of the vertebras. The pedicle screws are inserted such that their U-shaped seats are aligned with each other. The rod is positioned inside the slots, and the nuts are threaded onto the seats to secure the rods. A major problem with such an implant is that the nut is easily misaligned axially with the seat and cross threaded onto it. The threads on the seat and the nut may thus become damaged, and the entire implant must be replaced. Such designs may also have open or closed U-shaped channels. In closed designs, a screw hole is provided in the top portion to allow insertion of a set screw for tightening against the rod. In open designs, an oblong bridge with flanges or dovetails at the longitudinal ends is provided to allow locking of the ends of the bridge against the inner walls of the arms of the U-shaped brackets. This is typical of the Wisnewsky et al and Mellinger et al designs. A disturbing problem with these designs is the limited torque or tightening force that can be applied to the set screw because the transmitted force can spread apart or bend away the arms of the U-shaped bracket. The present invention seeks to correct this problem, without compromising ease of application, low profile, and strength of the assembly.
The objectives of the present spinal stabilization rod fastener are:
To develop a new mechanism for fastening a spinal stabilization rod for ease of application;
To eliminate the possibility of cross threading;
To have a low profile without sacrifing strength;
To prevent set screw forces from spreading the arms of U-shaped brackets of screw or hook assemblies;
To simplify design of instrumentation.
Further objectives of the present invention will become apparent from a consideration of the drawings and ensuing description.
A spinal stabilization rod fastener is comprised of an oval shaped rotatable bridge arranged to be connected across the arms of U-shaped brackets, the ends of which come to lie inside cavities or slots on the inner side of the arms. The bridge has a length which is greater than the width of the slot, and a width which is narrower than the slot. Arcuate rims are provided on the upper or superior side at the longitudinal ends of the bridge. The bridge is installed by aligning it longitudinally with the U- shaped slot or rod channel, positioning it between the slots or cavities in the arms, and rotating it to position its longitudinal ends in the cavities. After rotating it to 90 degrees, the lips on the upper edge of the slots overlap the arcuate rims. The outer edges of the slots are provided with margins to prevent displacement of the installed bridge in any direction as well as to increase the strength of the arm against forces tending to spread both arms upon tightening of the set screw. Such a set screw is arranged axially through the bridge for tightening against the rod. Rectangular slots are provided on each side of the screw hole for insertion of a flat screw driver or suitably shaped driving tool. The arcuate rims can be provided with elevations on their inner surface, on one half of the rim with a sharp edge which can lock onto a corresponding edge on the upper lip of the slots to prevent rotation of the bridge to more than ninety degrees.